In industrial processes where flammable or explosive materials are handled any leak or spill can cause an explosive and dangerous atmosphere. These conditions occur in many industrial environments, most typically in those involving petroleum and other chemicals, process gassers, metal and carbon dust, alcohol, grain, starch, flour, and fibers. To protect both personnel and plant, precautions should be taken within these hazardous areas. In the past, pneumatic controls have been used in these environments to avoid the risk of an electrical spark. Currently, while pneumatic equipment is still utilized, new technologies and engineering advances have created a wide range of electrical controls which allow far greater functionality, and still maintain a safe operating environment within such hazardous areas.
Many of these technologies, as they apply to process measurement and control, fall into an area of engineering known as “Intrinsic Safety.” Intrinsic Safety methodology describes a placement of an energy-limiting interface electrically between safe and hazardous areas. This energy-limiting interface and placement thereof restricts the electrical energy in the hazardous-area circuits so that potential electrical sparks or hot spots are too limited and weak to cause any ignition. The interface generally passes signals in both directions, but limits the voltage and current that can reach the hazardous area under particular fault conditions.
An intrinsic safety barrier is a device typically placed in a non-hazardous location or a safe location which permits the electrical interconnection of devices located in a hazardous area. In particular, the intrinsically safe barrier limits the power that can be introduced into the hazardous location to energy levels which are safe for the material being handled (or the process being performed) in such area. This barrier protects against, e.g., fault conditions such as shorting of the wires that are connected to the hazardous area side of the barrier by grounding the wires connected to the hazardous area side of such barrier therefore preventing a misconnection or failure of the power supply which allows an unsafe voltage to be applied to the safe area side of the barrier.
In a particular factory within which hazardous conditions exist, a conventional arrangement can be provided that includes intrinsically safe barrier which isolates a portion of the power grid of the factory from an array of sensors located throughout the factory. The sensors are located throughout the hazardous area of the factory. Each of the sensors is connected to the intrinsically safe barrier in order to receive power, and directly coupled to a computer processing system-via a communication link at the safe side of the barrier so as to communicate data and readings of the sensors thereto. In particular, this computer processing system receives the readings from each of the sensors of the sensor array through the associated communications link. While this system can be adequately used for measuring process parameters and variables throughout the factory, certain connections should be made between each of the sensors and the intrinsically safe barrier.
Certain publications relate to devices and systems utilizing particular barriers and safety devices. For example, U.S. Pat. No. 5,164,607 describes a fill sensor for a paint gun which provides a light sensor opposing a light source in a housing surrounding a transparent portion of a paint gun overflow line. An electric circuit is connected to an intrinsically safe barrier, and is adapted to operate through the intrinsically safe barrier for use in a manufacturing environment. The electric circuit senses whether there is paint in the overflow line. The electric circuit is electrically energized through the intrinsically safe barrier. The other side of the intrinsically safe barrier is connected to the relay. The relay may be any switch responsive to a predetermined electrical current, and provides an electrical isolation between the activating current carrying elements and the switched elements. The relay is responsive to the electrical circuit, and informs a user when a predetermined condition is sensed by the electrical circuit.
Another publication, i.e., U.S. Pat. No. 5,305,639, describes a liquid petroleum gas (LPG) gauge sensor unit that fits between the units of existing magnetically-coupled LPG gauge, and includes a magnetic field sensing switch, a mechanism to variably position the switch and an intermediate magnet. Upon sensing a particular orientation, the LPG gauge sensor transmits an indicative signal to an intrinsically safe barrier unit through cables and a waterproof junction box. The intrinsically safe barrier limits the power supplied to LPG gauge sensor. The indicative signal is sent to a fuel reordering system via another cable, and the fuel reordering system can then transmit a message to the distributor based on the particular orientation sensed by the LPG gauge sensor.
Furthermore, U.S. Pat. No. 6,021,162 describes a method and apparatus for decoding an encoded signal. The transmitter, as described in this publication, includes measurement circuitry and sensor circuitry. The measurement circuitry and the sensor circuitry are isolated by isolators. An isolation barrier (described in this publication as the isolators) are used to electrically isolate the sensor from the rest of the circuitry within the transmitter, and to prevent harmful electrical discharges. The sensor circuitry senses a process variable, and provides an output signal. The sensor circuitry frequency modulates process variable related signals to be transmitted across one of the isolators. The transmitter can be configured to communicate over a 4-20 mA current, as in the HART® protocol, or may be fully digital communications as in Fieldbus.
Also, U.S. Pat. No. 6,065,332 describes a method and apparatus for sensing and displaying the magnitude of torsional vibrations. As described in this publication, the current sensor senses the current of a motor driving a rotary table. In addition, the current sensor converts the magnetic flux produced by the current passing through the conductors in the power cord into a voltage signal, and delivers this voltage signal to a first intrinsically safe barrier. The signal passes from the first intrinsically safe barrier to a low pass filter, and then to a computer. An A/D converter, which is part of the computer, converts a digital signal that is representative of the voltage signal produced by the current sensor into an analog signal. The analog signal passes through a second barrier before reaching a first display, and then passes through a third intrinsically safe barrier before reaching a second display. The first and second displays provide the operator on a drill floor with information relating to the magnitude of a torsional vibration sensed by the apparatus.